Read this; from Oxfords website, I'm sure similar will apply to most.
Should I take more than three A-levels?
All courses at Oxford are very competitive, and offers are likely to be for three A grades at A-Level. Some candidates take a fourth or even fifth subject, as this can be one way of demonstrating the academic abilities that will be required for the intense studying of an Oxford degree.
However, taking extra A-Levels is certainly not the only way to demonstrate your abilities. Tutors would prefer a candidate who has read around their subject beyond their school or college work, and who shows a great passion for their subject, over a candidate who may have taken more subjects, but who is not able to discuss their interests with any enthusiasm or in any depth.
We also advise candidates not to spread themselves too thinly across too many subjects, where they may risk dropping a grade or two in their results. Three A grades would be sufficient to meet most conditional offers, but two As and two Bs would not.